1. Technical Field
The present invention relates generally to an improved method for treating a process fluid. More specifically, the present invention relates to an improved method for heating and cooling a slurry.
2. Field of the Invention
There are several hydrometallurgical processes that require a slurry to be heated to elevated temperature to cause a reaction or transformation to occur and subsequently be cooled to atmospheric conditions for further processing. Examples of such hydrometallurgical processes include recovery of nickel from nickel laterite ore, recovery of gold from refractory gold ore, and recovery of zinc from zinc concentrates. The process for recovery of nickel from laterite ore by leaching with sulfuric acid is called Pressure Acid Leach or PAL. The PAL process requires large volumes of thick abrasive slurry to be raised to elevated temperature, (about 240° C. or higher) and, consequently, to elevated pressures (about 50 atm). PAL process facilities typically have serious operating, maintenance, and reliability problems. Most of the problems can be attributed to the use of multiple pumping stations to raise the slurry to autoclave pressure, large amounts of flash and live injection steam to heat the slurry to or near autoclave temperature, and several steps of flash cooling to lower the temperature and pressure of the slurry down to atmospheric conditions.
The conventional facility for heating a slurry to elevated autoclave or reactor temperature and then cooling the slurry to atmospheric pressure conditions is composed of flash tanks (cooling) and heat exchangers (heating) to recover heat from the flash steam. The design of the recuperative heat exchangers may be by direct injection of flash steam, by shell and tube, or by some combination of the two types.
An example of the heat exchange systems used in design of conventional PAL facilities is shown in FIG. 1. An aqueous nickel ore slurry 1 is 25% to 50% by weight solids with or without the addition of sulfuric acid 2. The slurry 1 is at a temperature of less than 100° C. and is introduced into a series of heat exchangers 3 to preheat the slurry prior to leaching nickel from the ore in an autoclave 4 or a series of autoclaves. There is a pump 11 between the heat exchangers 3 and the autoclave 4. The heat exchangers 3 may be direct or indirect or a combination. Live steam 5 is used directly or indirectly to heat the slurry from the heat exchangers 3 to a temperature of 240° C. to 280° C. Sulfuric acid 2 may be added to the autoclave. The acidic nickel ore slurry is held in the autoclave 4 at temperature for about 20 minutes to 2 hours to cause about 95% of the nickel content of the ore (about 1% to 3% by weight) to go into solution. The treated slurry from the autoclave 4 enters a series of flash tanks 6 where the slurry is flash cooled to reduce the slurry temperature to its atmospheric boiling temperature at the discharge 7. The streams 8 to 10 from each flash tank 6, at succeeding lower temperature and pressure, are used to preheat the nickel ore slurry in the heat exchangers 3.
There are several problems with the flash cooling and recuperative heating process of FIG. 1. One problem is that the use of flash steam for injection steam heating requires a pump and pressure control facility at each stage of heating. The difficulties of pressure control, slurry level control and maintenance make this a troublesome means of recuperative heat exchange. In addition, injected flash steam increases the weight of slurry flowing, thereby increasing the amount of live steam needed to raise the slurry to autoclave or reactor temperature and the amount of excess flash steam (blow-off steam) wasted to the atmosphere.
Another problem is that the use of flash steam for indirect heating usually results in deposits on the heat exchange surface because of entrainment of slurry in the flash steam. This fouling lowers the overall heat transfer coefficient and the efficiency of the heat recovery. In addition, scale or deposits on the shell side of heat exchangers are difficult to clean and may not be completely removed on cleaning.
Yet another problem is that flash cooling from elevated temperatures requires several stages of depressurizing flash tanks to cool the slurry. High velocities (10 m/sec. and higher) occur in depressurizing a superheated slurry, resulting in severe abrasive wear of piping, level control valves and of other impinged surfaces.
An alternative to flash cooling would be to use counter-current heat exchangers to heat and cool the slurry. In this case, either the hot or cold slurry would be on the shell side of the heat exchanger, resulting in the problem of partly plugging by “sanding” (depositing larger particles) and/or scale deposits. Again, the problems are of loss of efficiency of heat transfer, difficulty in cleaning the shell side, and poor availability of the heat exchanger.